The present invention relates to metallic catalysts, and more particularly to metallic catalysts that promote catalytic reactions in a non-neutral pH liquid environment, with high leaching resistance, and the use thereof to promote chemical reactions in non-neutral liquid media.
Activated carbon has high chemical resistance in most inorganic and organic solutions. This is particularly so in acidic conditions. Other materials like alumina, silica, zeolite, and aluminosilicates, etc., do not fare as well.
This unique attribute of activated carbon has resulted in its commercial use as a catalyst support in non-neutral pH conditions. It has been found, however, that catalysts supported upon activated carbon surfaces are not retained on the surface of carbon during process under non-neutral pH conditions. This leaching phenomenon becomes a major concern in commercial use of these carbon-supported catalysts.
This commercial catalyst leachout problem is associated with a relatively weak chemical interaction between the carbon surface of the support and the catalyst. This has become a serious problem for the industry, of course, because catalyst leachout shortens the useful life of the catalyst and its productivity.
Therefore, a need has developed to provide an improved, activated, carbon-supported catalyst with resistance to leaching in non-neutral pH environments.
We have now discovered that metallic catalysts supported on porous, inorganic supports consisting of activated carbon-coated honeycombs of the type illustrated in U.S. Pat. Nos. 5,451,444 and 5,488,023 exhibit a surprising resistance to catalyst loss in both high and low pH reaction media. The present invention accordingly features a new family of carbon monolith-supported catalysts having substantially better stability in non-neutral pH solutions than previously obtained from commercially activated carbon-supported catalysts. This new family of carbon supported catalysts provides better catalytic performance, selectivity, and active lifetimes than prior art catalysts.
Therefore, in accordance with the present invention there is provided a method for promoting a catalytic reaction by contacting a liquid reactant mixture of non-neutral pH with a metallic catalyst supported by a solid inorganic support. The support includes a solid inorganic honeycomb substrate having surfaces with pores extending into the substrate, and incorporates a substantially uninterrupted adherent layer of activated carbon over the pore surfaces and an outer surface of the substrate. The metallic catalyst is supported upon, and/or dispersed throughout the adherent layer of activated carbon.
Generally speaking, the invention features carbon monolith-supported catalysts with high leaching resistance in catalytic chemical processes involving both strong acidic or basic conditions. The catalyst is formulated by loading the catalyst compositions onto carbon monolith supports using impregnation, precipitation, chemical vapor deposition, ion exchange, in-situ technology, and other techniques known in the art. These supported catalysts have surprisingly high resistance against leaching from their supports. The catalysts also have substantial differential advantages in catalyst performance: catalyst activity, selectivity, and stability in various reactions, as will be described hereinafter, particularly in comparison with commercially available, activated carbon supports and other inorganic supported catalysts.
Carbon monoliths are carbon impregnated honeycombs (CiHs) and extruded resin carbon honeycombs (ERCHs), illustrated in the aforementioned U.S. Pat. Nos. 5,451,444 and 5,488,023. These carbon monoliths are subjected to physical and chemical modifications to make their surface suitable for catalyst loading and to enhance their performance in practical applications. The modifications can be made through their composition formulations or through surface modifications after carbon formation. Depending upon the application, the carbon monoliths can be tailored so that their pore size range is in the micropore, mesopore, or macropore range, each with a desired pore size distribution. Mesopores are favored, when a chemical reaction is limited by a mass transfer step. On the other hand, when a reaction is governed by reaction kinetics, micropores are desirable. Carbon loading in a carbon monolith is usually in the range of between 5% to 99%. Pore volume and surface area are usually in the range of 0.1 to 1.4 ml/g C, and 10-3000 m2/g C, respectively.
Depending upon the catalyst, its composition, and the desired application, the preparation of the catalyst support will vary. For example, a Pt/CIH catalyst can be prepared using liquid solution impregnation and in-situ approaches; Ni/CIH can be prepared using co-precipitation and chemical vapor deposition. However, catalyst quality is measured by catalyst performance in a targeted application and its acidic or basic resistance, and/or leaching resistance, in solution.
Catalysts that are used for catalytic reactions in acidic or basic solution comprise precious group metals (Pt, Pd, Rh, Ru, Ir, Ag, Os and Re) and transitional metals (Fe, Co, Ni, Cu, Mo, Au, Cr, and Zn) and their compounds.
Catalysts additionally used for catalytic reactions in basic solution are alkali and alkaline metals (K, Na, Li and Mg, Ca, Ba, Ra), rare earth metals (La, Ce, Pr, Yb, and Ac) and their compounds.
Depending upon individual applications, some additives are needed, such as electron factor promoters and/or geometric factor promoters and/or carbon stabilizers. These additives enhance catalyst activity, selectivity, stability, lifetime, etc.
Electron factor promoters include alkali-metals and compounds, group IB metals and compounds, etc. Due to electron exchange capability with d-orbits in main catalysts, the promoters enhance main catalyst performance. Geometric factor promoters comprise alkali earth metals and compounds, rare earth metals and compounds, co-existing transitional metals and compounds thereof, etc.
These promoters are able to interact physically with the main catalyst, to keep catalyst particles apart during chemical processes. Thus, they maintain high surface area of the catalysts, and hence, high effective catalyst usage.
Carbon stabilizers include non-metal compounds like boron, silicon, phosphorous, sulfur, selenium, arsenic, and even oxygen, etc. Most carbon stabilizers can react with highly active carbon sites, in order to saturate carbon surfaces. Hence, they provide deactivation protection for the catalysts during storage and transportation.
The present invention features a new family of carbon monolith-supported catalysts. The unique family of carbon monolith-supported catalysts have substantial differential advantages in reactor design and operation, mass and heat transfer, intrinsic reaction kinetics, and catalyst performance, over commercially available, activated carbon and other supported catalysts in various industrial reactions. In particular, reactions carried out under acidic conditions are most significant as illustrated below:
1. Hydrogenation of nitrites and nitro-aromatics into amines are carried out in acidic solvent over carbon-supported precious metal catalysts. Acidic solvent in the reaction system has an additional promotion function to the main catalyst.
2. Purification of xcex5-caprolactam (nylon-6 precursor) by selectively hydrogenating olefinic impurities at a 10 to 50 ppm level is conducted in a strong acidic (pH=1 to 6) condition, originating from synthesis of xcex5-caprolactam in one of three strong acids (sulfuric acid, nitric acid, and hydrochloric acid), depending upon the process.
3. Hydrogenation of aromatic and paraffinic aldehydes, ketones, and acids into their corresponding alcohols is performed in acidic solvents to prevent hydrogenolysis and to promote catalyst performance.
4. Acidic solvents are often used to promote the reaction or prevent side reactions in hydrogenation of other organic compounds containing unsaturated double bonds of Cxe2x95x90C, Cxe2x95x90O, Cxe2x95x90N, and Nxe2x95x90O, unsaturated triple bonds of Cxe2x89xa1C, Cxe2x89xa1N, aromatic rings, and benzyl compounds, etc.
5. Acetoxylation of olefins and toluene into their corresponding acetates is favored, when conducted in hydrochloric acid over promoted precious metal catalysts in a temperature range of approximately 25 to 200xc2x0 C.
6. Carbonylation of methanol to acetic acid and its derivatives involves the highly corrosive solvent CH3I, and causes leaching of catalyst from catalyst supports. This is one of the key issues related to the practicality of heterogeneous catalysis for methanol carbonylations. In most of hydrogenolysis reactions, acidic solvent can promote catalytic performance over carbon-supported precious metal catalysts.
Commercial activated carbon and other supported catalysts in various industrial basic reactions are:
1. Purification of xcex5-caprolactam (nylon-6 precursor) by selectively hydrogenating olefinic impurities at a 10-50 ppm level is conducted in a strong basic (pH=7.5 to 14) condition originated from neutralization of one of three strong acids after synthesis of xcex5-caprolactam.
2. Hydrogenation of nitroaromatic compounds to hydrazobenzenes is more effective when the reaction is carried out in organic and inorganic base media over supported precious metal catalysts in a temperature range from ambient to 100xc2x0 C.
3. Dehydrohalogenation of aliphatic and aromatic halocompounds into their corresponding non-halogenated hydrocarbons is more selective in basic solvents, since the formed by-product, hydrohalogenic acid, can be removed easily in the reaction through acid-base reaction with a reaction solvent.
4. Selective hydrogenations of nitro-olefins to oximes are more effective over supported Pd catalysts, when basic solvents like pyridine are used.
In addition, it is believed that the catalysts and catalyst supports of this invention can be advantageous, when used in the following catalytic reactions:
a) hydrogenation of alkynes into olefins, olefins into alkanes, ketones and aldehydes into alcohols, aromatics into cycloparaffins, fatty oil purifications, etc.; and
b) hydrotreating crude oils to remove sulfur (hydrodesulfurization, HDS), nitrogen (hydrodenitrogenation, HDN), and halogen (hydrodehalogenation, HDX); carbonylation of methanol to acetic acid and its derivatives.